1. Field of the Invention
The present invention relates to a projection exposure method and a projection exposure apparatus. In particular, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by projecting an image of a pattern formed on a mask onto the photosensitive substrate by the aid of a projection optical system. Especially, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by overlaying and transferring images of patterns formed on a plurality of masks onto a predetermined area on the photosensitive substrate.
2. Description of the Related Art
Various exposure apparatuses have been hitherto used, for example, when semiconductor elements or liquid crystal display elements are produced by means of the photolithography step. At present, a projection exposure apparatus is generally used, in which an image of a pattern formed on a photomask or reticle (hereinafter generally referred to as xe2x80x9creticlexe2x80x9d) is transferred via a projection optical system onto a substrate (hereinafter referred to as xe2x80x9cphotosensitive substratexe2x80x9d, if necessary) such as a wafer or a glass blade applied with a photosensitive material such as photoresist on its surface. In recent years, a reduction projection exposure apparatus (so-called stepper) based on the so-called step-and-repeat system is predominantly used as the projection exposure apparatus, in which a photosensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the photosensitive substrate is moved in a stepwise manner (subjected to stepping) by using the substrate stage to repeat the operation for successively exposing respective shot areas on the photosensitive substrate with the image of the pattern formed on the reticle.
Recently, a projection exposure apparatus based on the step-and-scan system (scanning type exposure apparatus as described, for example, in Japanese Laid-Open Patent Publication No. 7-176468 corresponding to U.S. Pat. No. 5,646,413), which is obtained by applying modification to the stationary type exposure apparatus such as the stepper, is also used relatively frequently. The projection exposure apparatus based on the step-and-scan system has, for example, the following merits. That is, (1) the projection optical system is easily produced because a large field can be exposed by using a smaller optical system as compared with the stepper, and a high throughput can be expected owing to the decrease in number of shots because a large field is exposed. Further, (2) an averaging effect is obtained owing to relative scanning for the reticle and the wafer with respect to the projection optical system, and it is possible to expect improvement in distortion and depth of focus. Moreover, it is considered that the scanning type projection exposure apparatus will be predominantly used in place of the stepper, because a large field will become essential in accordance with the increase in the degree of integration of the semiconductor element, which is 16 M (mega) at present and will become 64 M for DRAM, 256 M, and 1 G (giga) in future as the progress proceeds along with times.
When the photosensitive substrate is subjected to exposure by using the projection exposure apparatus as described above, it has been contemplated to improve the resolution and the depth of focus for a pattern to be formed, by using the modified illumination method, for example, the SHRINC (Super High Resolution by Illumination Control) method as described in Japanese Laid-Open Patent Publication No. 4-273245.
Such a projection exposure apparatus is principally used as a mass-production machine for semiconductor elements or the like. Therefore, the projection exposure apparatus necessarily required to have a processing ability that how many sheets of wafers can be subjected to the exposure process for a certain period of time. That is, it is necessarily required for the projection exposure apparatus to improve the throughput.
In this context, in the case of the projection exposure apparatus based on the step-and-scan system, when a large field is exposed, the improvement in throughput is expected because the number of shots to be exposed on the wafer is decreased as described above. However, since the exposure is performed during movement at a constant velocity in accordance with synchronized scanning for the reticle and the wafer, it is necessary to provide acceleration and deceleration areas before and after the constant velocity movement area. If a shot having a size equivalent to a shot size of the stepper is exposed, there is a possibility that the throughput is rather decreased as compared with the stepper.
The outline of the flow of the process in such a projection exposure apparatus is as follows.
(1) At first, a wafer load step is performed, in which a wafer is loaded on a wafer table by using a wafer loader.
(2) Next, a search alignment step is performed, in which the position of the wafer is roughly detected by using a search alignment mechanism. Specifically, the search alignment step is performed, for example, on the basis of the contour of the wafer, or by detecting a search alignment mark on the wafer.
(3) Next, a fine alignment step is performed, in which the position of each of the shot areas on the wafer is accurately determined. In general, the EGA (enhanced global alignment) system is used for the fine alignment step. In this system, a plurality of sample shots included in the wafer are selected beforehand, and positions of alignment marks (wafer marks) affixed to the sample shots are successively measured. Statistical calculation based on, for example, the so-called least square method is performed on the basis of results of the measurement and designed values of the shot array to determine all shot array data on the wafer (see, for example, Japanese Laid-Open Patent Publication No. 61-44429 corresponding to U.S. Pat. No. 4,780,617). In this system, it is. possible to relatively accurately determine the coordinate positions of the respective shot areas at a high throughput.
(4) Next, an exposure step is performed, in which the image of the pattern on the reticle is transferred onto the wafer via the projection optical system while successively positioning the respective shot areas on the wafer to be located at exposure positions on the basis of the coordinate positions of the respective shot areas having been determined in accordance with the EGA system or the like described above and the previously measured baseline amount.
(5) Next, a wafer unload step is performed, in which the wafer on the wafer table having been subjected to the exposure process is wafer-unloaded by using a wafer unloader. The wafer unload step is performed simultaneously with the wafer load step (1) described above in which the exposure process is performed. That is, a wafer exchange step is constructed by the steps (1) and (5).
As described above, in the conventional projection exposure apparatus, the roughly classified four operations are repeatedly performed by using one wafer stage, i.e., wafer exchangexe2x86x92search alignmentxe2x86x92fine alignmentxe2x86x92exposurexe2x86x92wafer exchange.
The throughput THOR [sheets/hour] of such a projection exposure apparatus can be represented by the following expression (1) assuming that the wafer exchange time is T1, the search alignment time is T2, the fine alignment time is T3, and the exposure time is T4.
THOR=3600/(T1+T2+T3+T4)xe2x80x83xe2x80x83(1)
The operations of T1 to T4 are executed repeatedly and successively (sequentially) as in T1xe2x86x92T2xe2x86x92T3xe2x86x92T4xe2x86x92T1. . . . Accordingly, if the individual elements ranging from T1 to T4 involve high speeds, then the denominator is decreased, and the throughput THOR can be improved. However, as for T1 (wafer exchange time) and T2 (search alignment time), the effect of improvement is relatively small, because only one operation is performed for one sheet of wafer respectively. As for T3 (fine alignment time), the throughput can be improved if the sampling number of shots is decreased in the case of the use of the EGA system, or if the measurement time for a single shot is shortened. However, on the contrary, the alignment accuracy is deteriorated. Therefore, it is impossible to easily shorten T3.
On the other hand, T4 (exposure time) includes the wafer exposure time and the stepping time for movement between the shots. For example, in the case of the scanning type projection exposure apparatus based on, for example, the step-and-scan system, it is necessary to increase the relative scanning velocity between the reticle and the wafer in an amount corresponding to the reduction of the wafer exposure time. However, it is impossible to easily increase the scanning velocity because the synchronization accuracy is deteriorated.
Important conditions for such a projection exposure apparatus other than those concerning the throughput described above include (1) the resolution, (2) the depth of focus (DOF: Depth of Focus), and (3) the line width control accuracy. Assuming that the exposure wavelength is xcex, and the numerical aperture of the projection lens is N.A. (Numerical Aperture), the resolution R is proportional to xcex/N.A., and the depth of focus (DOF) is proportional to xcex/(N.A.)2.
Therefore, in order to improve the resolution R (in order to decrease the value of R), it is necessary to decrease the exposure wavelength xcex, or it is necessary to increase the numerical aperture N.A. Especially, in recent years, semiconductor elements or the like have developed to have high densities, and the device rule is not more than 0.2 xcexcm L/S (line and space). For this reason, a KrF excimer laser is used as an illumination light source in order to perform exposure for the pattern. However, as described above, the degree of integration of the semiconductor element will be necessarily increased in future. Accordingly, it is demanded to develop an apparatus provided with a light source having a wavelength shorter than that of KrF. Representative candidates for the next generation apparatus provided with the light source having the shorter wavelength as described above include, for example, an apparatus having a light source of ArF excimer laser, and an electron beam exposure apparatus. However, the case of the ArF excimer laser involves numerous technical problems in that the light is scarcely transmitted through a place where oxygen exists, it is difficult to provide a high output, the service life of the laser is short, and the cost of the apparatus is expensive. The electron beam exposure apparatus is inconvenient in that the throughput is extremely lowered as compared with the light beam exposure apparatus. In reality, the development of the next generation machine, which is based on the principal viewpoint of the use of a short wavelength, does not proceed so well.
It is conceived to increase the numerical aperture N.A., as another method to increase the resolution R. However, if N.A. is increased, there is a demerit that DOF of the projection optical system is decreased. DOF can be roughly classified into UDOF (User Depth of Focus: a part to be used by user: for example, difference in level of pattern and resist thickness) and the overall focus difference of the apparatus itself. Up to now, UDOF has contributed to DOF in a greater degree. Therefore, the development of the exposure apparatus has been mainly directed to the policy to design those having a large DOF. Those practically used as the technique for increasing DOF include, for example, modified illumination.
By the way, in order to produce a device, it is necessary to form, on a wafer, a pattern obtained by combining, for example, L/S (line and space), isolated L (line), isolated S (space), and CH (contact hole). However, the exposure parameters for performing optimum exposure differ for every shape of the pattern such as L/S and isolated line described above. For this reason, a technique called ED-TREE (except for CH concerning a different reticle) has been hitherto used to determine, as a specification of the exposure apparatus, common exposure parameters (for example, coherence factor a, N.A., exposure control accuracy, and reticle drawing accuracy) so that the resolution line width is within a predetermined allowable error with respect to a target value, and a predetermined DOF is obtained. However, it is considered that the following technical trend will appear in future.
(1) In accordance with the improvement in process technology (improvement in flatness on the wafer), the difference in pattern level will be progressively lowered, and the resist thickness will be progressively decreased. There will be a possibility that the UDOF may change from an order of 1 xcexcmxe2x86x920.4 xcexcm.
(2) The exposure wavelength changes to be short, i.e., g-ray (436 nm)xe2x86x92i-ray (365 nm)xe2x86x92KrF (248 nm). However, investigation will be made for only a light source based on ArF (193 nm) in future. Further technical hurdle is high. Thereafter, the progress will proceed to EB exposure.
(3) It is expected that the scanning exposure such as those based on the step-and-scan system will be predominantly used for the stepper, in place of the stationary exposure such as those based on the step-and-repeat system. The step-and-scan system makes it possible to perform exposure for a large field by using a projection optical system having a small diameter (especially in the scanning direction), in which it is easy to realize high N.A. corresponding thereto.
In the background of the technical trend as described above, the double exposure method is reevaluated as a method for improving the limiting resolution. Trial and investigation are made such that the double exposure method will be used for KrF exposure apparatus and ArF exposure apparatus in future to perform exposure up to those having 0.1 xcexcm L/S. In general, the double exposure method is roughly classified into the following three methods.
(1) L/S""s and isolated lines having different exposure parameters are formed on different reticles, and exposure is performed for each of them on an identical wafer under an optimum exposure condition.
(2) For example, when the phase shift method is introduced, L/S has a higher resolution at an identical DOF as compared with the isolated line. By utilizing this fact, all patterns are formed with L/S""s by using the first reticle, and L/S""s are curtailed for the second reticle to form the isolated lines.
(3) In general, when the isolated line is used, a high resolution can be obtained with a small N.A. as compared with L/S (however, DOF is decreased). Accordingly, all patterns are formed with isolated lines, and the isolated lines, which are formed by using the first and second reticles respectively, are combined to form L/S""s.
The double exposure method described above has two effects of improvement in resolution and improvement in DOF.
However, in the double exposure method, the exposure process must be performed several times by using a plurality of reticles. Therefore, inconveniences arise in that the exposure time (T4) is not less than two fold as compared with the conventional apparatus, and the throughput is greatly deteriorated. For this reason, actually, the double exposure method has not been investigated so earnestly. The improvement in resolution and depth of focus (DOF) has been hitherto made by means of, for example, the use of an ultraviolet exposure wavelength, modified illumination, and phase shift reticle.
However, even when the contrivance is made, for example, for the exposure wavelength made to be ultraviolet, the modified illumination, and the phase shift reticle as described above, it is difficult to realize a resolution line width of 0.1 xcexcm which is the target for the next generation machine. Therefore, it is doubtless that the realization of exposure up to 0.1 xcexcm L/S based on the use of the aforementioned double exposure method for the KrF or ArF exposure apparatus makes a prevailing choice for the development of the next generation machine aimed at mass production of 256 M or 1 G DRAM.
It is conceived, as a means for further improving the resolving power based on the use of the double exposure method, to perform the double exposure by using the modified illumination as described above. In the case of the conventional modified illumination method, it is possible to improve the resolution and the depth of focus (DOF) for L/S and isolated L in a predetermined direction. However, the conventional modified illumination method is inconvenient in that the resolution and the depth of focus are considerably deteriorated for a pattern disposed in a direction perpendicular to the predetermined direction. Almost all such inconveniences can be dissolved by simultaneously performing modified illumination in two orthogonal axial directions. However, when each L pattern is inspected, the image is markedly deteriorated (for example, the edge portions become dull and tapered) at both end portions of the pattern (portions at which the two-dimensional edge exists). Therefore, an inconvenience arises in that the improvement in accuracy cannot be expected all the more as compared with a case in which exposure is performed by using a zonal type illumination method.
The double exposure method involves another inherent problem. That is, this method suffers from an inconvenience that the throughput is necessarily lowered, because it is necessary to perform the exposure process two times or more by using a plurality of reticles as described above. In this case, the inconvenience is not limited only to the increase in actual exposure time. In the conventional technique, since one sheet of reticle is placed on the reticle stage, the following inconveniences also arise when the double exposure method is carried out. That is, the throughput is lowered in an amount corresponding to the necessity to successively repeat a series of sequence in which (1) the reticles stored in a reticle library are taken out one by one by using a reticle loader or the like to perform reticle exchange with respect to the reticle stage, (2) the reticle is subjected to positional adjustment (alignment), (3) the exposure process is thereafter performed by using the reticle, and then the process routine returns to the step (1) again to exchange the reticle. Therefore, it is demanded to improve the throughput even to some extent by shortening the exchange time for the reticle when a plurality of reticles are exchanged and used.
It is conceived, as a method for shortening the reticle exchange time, to place a plurality of reticles on a reticle stage. However, if such a method is adopted, an inconvenience also arises in that the stage becomes to have a large size, and its positional control performance is deteriorated especially in the case of a scanning type exposure apparatus.
Further, it is necessary to make a special design concerning the management of the plurality of reticles, when the plurality of reticles are used as a set as in the double exposure method as described above.
The present invention has been made taking such circumstances into consideration. An object of the present invention is to provide a projection exposure method which especially makes it possible to expose a photosensitive substrate with a desired image of a pattern at a high resolution and with a large depth of focus.
Another object of the present invention is to provide a scanning type projection exposure apparatus which especially makes it possible to accurately control the position of a mask stage which carries a mask thereon.
Still another object of the present invention is to provide a projection exposure apparatus which especially makes it possible to decrease the foot print of the main apparatus body while improving the throughput in mask exchange and the control performance of a mask stage.
Still another object of the present invention is to provide a projection exposure apparatus which especially makes it possible to easily perform the management for a plurality of masks when the plurality of masks are used as a set.
According to a first aspect of the present invention, there is provided a projection exposure method for exposing a predetermined area on a photosensitive substrate (W1 or W2) by overlaying and transferring a plurality of patterns via a projection optical system (PL) to the predetermined area on the photosensitive substrate, the projection exposure method comprising:
a first exposure step of arranging a mask (R1) formed with a first pattern (RP1) composed of a line pattern disposed along a predetermined direction, at a position conjugate to the photosensitive substrate (W1 or W2) in relation to the projection optical system (PL), and exposing the predetermined area on the photosensitive substrate by illuminating the first pattern with an illumination light beam (L1) which is inclined by a predetermined amount in a direction perpendicular to the line direction of the first pattern (RP1) with respect to an optical axis (AX) of the projection optical system; and
a second exposure step of arranging a mask (R2) formed with a second pattern (RP2) composed of a line pattern disposed along a direction perpendicular to the first pattern, at the position conjugate to the photosensitive substrate (W1 or W2) in relation to the projection optical system, and exposing the predetermined area on the photosensitive substrate by illuminating the second pattern (PR2) with an illumination light beam which is inclined by a predetermined amount in a direction perpendicular to the line direction of the second pattern (RP2) with respect to the optical axis (AX).
According to the present invention, the first pattern is illuminated in the first exposure step with the illumination light beam which is inclined in the predetermined amount in the direction perpendicular to the line direction of the first pattern with respect to the optical axis of the projection optical system. The illumination is so-called modified illumination (two-beam illumination or two-beam image formation), in which the 0-order diffracted light beam and, for example, the xe2x88x921-order diffracted light beam generated from the first pattern on the mask are symmetrical relative to the optical axis, and only the two light beams pass through-the projection optical system, wherein the +1-order diffracted light beam does not pass therethrough (See FIG. 15). Accordingly, the two-beam interference does not cause any wave front aberration on the photosensitive substrate. Therefore, it is possible to form an image of the pattern having a high resolution in the line direction and having a large depth of focus. The modified illumination method such as those based on the two-light beam illumination has been already known, for which, for example, reference may be made to U.S. Pat. No. 5,638,211.
Similarly, in the second exposure step, it is possible to form, in the line direction, an image of the pattern having a high resolution and having a large depth of focus, by performing the modified illumination for the mask formed with the second pattern composed of the line pattern disposed in the direction perpendicular to the first pattern. As described above, the first pattern and the second pattern, which are in the orthogonal relationship, are separately subjected to the illumination based on the use of the modified illumination suitable for the respective patterns. Thus, it is possible, for each line direction, to form the image of the pattern having the high resolution and having the large depth of focus.
In order to perform the modified illumination as described above, the first exposure step may be carried out such that the first pattern is illuminated by transmitting the illumination light beam through two eccentric areas each having a center at a position of point symmetry relative to the optical axis in a plane substantially corresponding to one obtained by Fourier transformation for a mask plane in relation to an illumination system for generating the illumination light beam or in a plane in the vicinity thereof. Further, the second exposure step may be carried out such that the second pattern is illuminated by transmitting the illumination light beam through two eccentric areas each having a center at a position of point symmetry in relation to the optical axis in a plane substantially corresponding to one obtained by Fourier transformation for a mask plane in relation to the illumination system or in a plane in the vicinity thereof.
It is desirable that the second pattern has a line pattern which extends in the direction perpendicular to the first pattern so that the second pattern overlaps both line end portions of the first line pattern. That is, when the illumination is performed in accordance with the modified illumination in conformity with the line pattern in the predetermined direction, it is possible to form a pattern image having a high resolution and having a large depth of focus in the line direction, however, conversely, the depth of focus is decreased, and the resolution is deteriorated (the edge portion becomes dull and tapered) at pattern portions (both end edges of the line pattern) perpendicular to the line direction. For this reason, the second pattern is subjected to overlay exposure at least for the both end portions of the pattern formed by using the first pattern. Thus, it is possible to obtain a pattern image after development, in which exposure-defective portions at the both line ends of the first pattern are removed by the second pattern. As a result, it is possible to obtain a pattern image having no exposure-defective portion.
The pattern image having the high resolution can be obtained, for example, for line and space patterns and isolated line patterns, only by performing the overlay exposure (double exposure) comprising the first and second exposure steps described above. However, the projection exposure method may further comprise a third exposure step of illuminating a mask formed with a third pattern to remove a specific portion constituting an exposure pattern which has been obtained on the predetermined area on the photosensitive substrate by means of the first and second exposure steps. In this embodiment, the third pattern may be illuminated after the second exposure step with an illumination light beam obtained by transmitting the illumination light beam from a light source through a zonal area centered about the optical axis in a plane substantially corresponding to one obtained by Fourier transformation for a mask plane on which the third pattern is formed or in a plane in the vicinity thereof. The inclusion of the third exposure step makes it possible, for example, to obtain, after development, a pattern image having a high resolution comprising patterns from which, for example, specified portions for forming the third pattern are removed, by performing the overlay exposure under the condition of zonal illumination based on the use of the mask formed with the third pattern, from the pattern image having the high resolution and having the large depth of focus formed as a result of the overlay exposure carried out in the first and second exposure steps. This process is preferred, for example, when an contact hole image or the like is formed.
In order to execute the first to third exposure steps, it is possible to use a mask stage for carrying the mask formed with the first pattern, the mask formed with the second pattern, and the mask formed with the third pattern, and a substrate stage for carrying the photosensitive substrate. The identical area on the substrate may be subjected to triple exposure by scanning the respective masks across the illumination light beam by synchronously moving the mask stage and the substrate stage with respect to the illumination light beam.
According to a second aspect of the present invention, there is provided a scanning type exposure apparatus for exposing a photosensitive substrate (W1 or W2) with a pattern formed on a mask (R) by scanning the mask across an illumination light beam while synchronously moving the mask (R) and the photosensitive substrate (W1 or W2) with respect to the illumination light beam, the scanning type exposure apparatus comprising a mask stage (RST) which is movable in an in plane direction of the mask while carrying a plurality of masks (R3, R4, R5) and which has a reflective surface (262) formed on a side of the mask stage extending in a scanning direction; a detecting system (30) which detects a position of the mask stage by irradiating the reflective surface (262) with a light beam; a memory (91) for storing previously measured surface curvature data on the reflective surface (262) in relation to positions of the plurality of masks (R3, R4, R5) to be carried; and a control unit (90) which controls the position of the mask stage (RST) on the basis of the surface curvature data on the reflective surface (262) stored in the memory (91).
According to the scanning type exposure apparatus of the present invention, the control unit controls the position of the mask stage in the in plane direction of the mask including the scanning direction and the direction perpendicular to the scanning direction on the basis of the surface curvature data on the reflective surface of the mask stage corresponding to the respective masks stored in the memory (corresponding to the position on the mask stage for carrying each of the masks thereon). Therefore, even if any curvature is generated on the reflective surface due to deformation caused, for example, by self-weight of the mask stage, it is possible to highly accurately control the position of the mask stage during scanning without being affected thereby. That is, the scanning exposure can be executed while accurately controlling the position of the mask with respect to the photosensitive substrate. When the plurality of masks are arranged in the scanning direction and carried on the mask stage, the length of the mask stage in the scanning direction is long. For this reason, the reflective surface is further apt to be bent or curved, and thereby the detection error tends to occur for the position of the mask. The scanning type exposure apparatus according to the present invention is effective in such a case. For example, when three sheets of masks are carried on the mask stage, triple exposure can be executed by repeatedly scanning and exposing an identical area on the photosensitive substrate with patterns of the respective masks. Therefore, the scanning type exposure apparatus according to the second aspect of the present invention is extremely useful for the projection exposure method according to the first aspect.
According to a third aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate by projecting images of patterns formed on a plurality of masks (for example, 316, 318), formed by a projection optical system (PL), onto a photosensitive substrate (W1 or W2) respectively, the projection exposure apparatus comprising:
a first mask stage (312) which is movable in a two-dimensional plane while carrying a first mask (316); a second mask stage (314) which is movable independently from the first mask stage (312) on the same plane as that for the first mask stage (312) while carrying a second mask (318); a mark-detecting system (326L1, 326R1, 326L2, 326R2) which is provided separately from the projection optical system (PL),and detects marks formed on the first mask (316) and the second mask (318); a transport system (322, 324) which delivers the mask (for example, 316, 318) with respect to the first mask stage (312) and the second mask stage (314); and a control unit (90) which controls the first mask stage (312), the second mask stage (314), and the transport system (322, 324) respectively so that exposure is performed by using the mask (for example, 318) on any one of the mask stages of the first mask stage (312) and the second mask stage (314), during which at least one of mask exchange effected by the transport system (322) and mark detection effected by the mark-detecting system (326L1, 326R1) is performed on the other mask stage (for example, 312).
In the projection exposure apparatus according to the third aspect of the present invention, the first mask stage for carrying the first mask thereon and the second mask stage for carrying the second mask thereon are movable independently from each other. The operations of the first mask stage, the second mask stage, and the transport system are controlled by the control unit so that the exposure is performed by the aid of the projection optical system by using the mask on any one of the mask stages, during which at least one of the mark detection for the mask effected by the mark-detecting system such as a mask alignment system and the mask exchange effected by the transport system is concurrently processed in parallel to one another on the other mask stage. As a result, it is possible to improve the throughput as compared with a case in which a single mask stage is used to successively and sequentially perform mask exchange, mask mark detection, and exposure on the mask stage after completion of exposure for a mask on the mask stage.
In the projection exposure apparatus according to the third aspect, the mark-detecting system may further comprise, for example, a first mask alignment system (326L1, 326R1) which detects the mark on the first mask (316) placed on the first mask stage (312), and a second mask alignment system (236L2, 326R2) disposed on a side opposite to the first mask alignment system (326L1, 326R1) in relation to the projection optical system (PL) in a direction of a first axis along which the projection optical system (PL) and the first mask alignment system (326L1, 326R1) are arranged, and detects the mark on the second mask (318) placed on the second mask stage (314). In this embodiment, the projection exposure apparatus may further comprise an interferometer system (BI11Y, BI12Y, BI13X, BI14X, BI15X) having a first length-measuring axis (BI11Y) for always measuring a position of the first mask stage (312) in the direction of the first axis from one side in the direction of the first axis, a second length-measuring axis (BI12Y) for always measuring a position of the second mask stage (314) in the direction of the first axis from the other side in the direction of the first axis, a third length-measuring axis (BI13X) which perpendicularly intersects the first axis at an exposure position of the projection optical system (PL), a fourth length-measuring axis (BI14X) which perpendicularly intersects the first axis at a detection position of the first mask alignment system (326L1, 326R1), and a fifth length-measuring axis (BI15X) which perpendicularly intersects the first axis at a detection position of the second mask alignment system (326L2, 326R2). The interferometer system can measure two-dimensional positions of the first mask stage (312) and the second mask stage (314) respectively by using the length-measuring axes.
In this embodiment, the control unit (90) is desirably operated such that an-interferometer having the third length-measuring axis (BI13X) is reset in a state in which positional measurement is executable for the first mask stage (312) based on the use of a measured value obtained by using the third length-measuring axis (BI13X) when the first mask stage (312) is moved to the exposure position from a position at which management of the first mask stage is performed based on the use of a measured value obtained by using the fourth length-measuring axis (BI14X) of the interferometer system (BI11Y, BI12Y, BI13X, BI14X, BI15X), and an interferometer having the fourth length-measuring axis (BI14X) is reset in a state in which positional measurement is executable for the first mask stage (312) based on the use of a measured value obtained by using the fourth length-measuring axis (BI14X) when the first mask stage (312) is moved to an alignment position from a position at which management of the first mask stage is performed based on the use of a measured value obtained by using the third length-measuring axis (BI13X); and the interferometer having the third length-measuring axis (BI13X) is reset in a state in which positional measurement is executable for the second mask stage (314) based on the use of a measured value obtained by using the third length-measuring axis (BI13X) when the second mask stage (314) is moved to the exposure position from a position at which management of the second mask stage is performed based on the.use of a measured value obtained by using the fifth length-measuring axis (BI15X) of the interferometer system (BI11Y, BI12Y, BI13X, BI14X, BI15X), and an interferometer having the fifth length-measuring axis (BI15X) is reset in a state in which positional measurement is executable for the second mask stage (314) based on the use of a measured value obtained by using the fifth length-measuring axis (BI15X) when the second mask stage (314) is moved to an alignment position from a position at which management of the second mask stage is performed based on the use of a measured value obtained by using the third length-measuring axis (BI13X).
When the control is made as described above, the exposure is performed by using the mask on one of the mask stages by the aid of the projection optical system located at the center (exposure operation), during which the mark detection is performed for the mask on the other mask stage by using one mask alignment system (alignment operation). When the exposure operation and the alignment operation are changed, then the one mask stage having been located under the projection optical system can be easily moved to the position for the other mask alignment system, the other mask stage having been located at the position for the one mask alignment system can be easily moved up to the position under the projection optical system, only by moving the two mask stages along the direction of the first axis toward the other mask alignment system. The positional detection is performed for the positions of the respective mask stages by using the interferometers. In this process, even when the mask stage is moved passing over the length-measuring axes of the interferometers disposed to be directed to the respective positions of the projection optical system and the mask alignment system, the positional measurement is executable at the respective positions for the projection optical system and the mask alignment system by resetting the interferometer. When the plurality of masks are successively used as described above, it is possible to alternately use the two mask alignment systems so that the exposure operation and the alignment operation are concurrently dealt with in parallel to one another. In this embodiment, it is sufficient to independently measure the position of the mask stage by using the third, fourth, and fifth length-measuring axes at their measurement areas owing to the interferometer-resetting function. Accordingly, the mask stage can be miniaturized, and it is allowed to have a light weight. Specifically, it is sufficient for each of the mask stages to have a size of a degree which is somewhat larger than the reticle.
According to a fourth aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate by projecting images of patterns formed on a plurality of masks (for example, R1, R2), formed by a projection optical system (PL), onto a photosensitive substrate (W1 or W2) respectively, the projection exposure apparatus comprising at least one mask-accommodating container formed with a plurality of accommodating areas for accommodating the plurality of masks (R1, R2) respectively; and a mask library (for example, 220) for accommodating the at least one mask-accommodating container.
In the exposure apparatus according to the fourth aspect, for example, when the plurality of masks are used as a set as in the double exposure, the masks of an amount corresponding to a predetermined number of sheets can be independently accommodated in one accommodating container respectively. The operation of taking out and inserting the accommodating container with respect to the mask library can be performed as one operation. Moreover, the combination of the masks is scarcely mistaken when the plurality of masks are stored. Thus, it is possible to easily manage the plurality of masks.
According to a fifth aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate by projecting images of patterns formed on a plurality of masks (for example, R1, R2), formed by a projection optical system (PL), onto a photosensitive substrate (W1 or W2) respectively, the projection exposure apparatus comprising a plurality of individual accommodating containers (212, 214) for individually accommodating the plurality of masks (for example, R1, R2) respectively; a fixing tool (216) for superimposing and integrating the plurality of accommodating containers into one unit; and a mask library (220) for accommodating the plurality of individual accommodating containers integrated into the one unit by the aid of the fixing tool.
In the projection exposure apparatus according to the fifth aspect, for example, when the plurality of masks are used as a set, then the respective masks are accommodated in the individual accommodating containers respectively, and the plurality of individual accommodating containers can be united and fixed by using the fixing tool. Accordingly, for example, the plurality of masks can be conveyed in the fixed state, or they can be accommodated in the mask library as they are. Thus, it is possible to easily manage the plurality of masks as set units. When the plurality of individual accommodating containers are fixed by using the fixing tool, they form a single individual accommodating container unit in which the masks are individually accommodated. Accordingly, this arrangement is advantageous in that the conventional dust-measuring mechanism for measuring dust on the mask can be utilized as it is. Therefore, it is possible to easily manage the masks even when the exposure is performed by using the plurality of masks.
In this aspect, the fixing tool may serve to directly superimpose the plurality of individual accommodating containers. However, the fixing tool may be a coupling tool (238a, 238b) for coupling the individual accommodating containers (for example, 232, 234, 236) in a superimposing direction while being separated by a predetermined spacing distance. The mask library may comprise a plurality of support sections (244a to 244f) for supporting both side ends of the plurality of individual accommodating containers, and the plurality of support sections may be provided at a spacing distance corresponding to a thickness of the individual accommodating container. Further, the coupling tool may be attached to a portion of the individual accommodating container to make no interference with the support section. The structures of the mask library and the coupling tool designed as described above make it possible to use the conventional mask library for the cassette for individually accommodating masks as it is. Thus, the plurality of individual accommodating containers integrated into one unit by the aid of the fixing tool can be supported respectively by using the plurality of support sections provided therein. For example, the coupling tool may be attached to only a central portion of the individual accommodating container.
According to a sixth aspect of the present invention, there is provided a projection exposure apparatus comprising a projection exposure apparatus for exposing a photosensitive substrate by projecting images of patterns formed on a plurality of masks onto the photosensitive substrate respectively, the projection exposure apparatus comprising:
a first mask-driving system which moves a first mask in a predetermined plane;
a second mask-driving system which moves a second mask independently from the first mask, in a plane which is the same as that for the first mask or which is parallel thereto; and
a control system which controls the first mask-driving system and the second mask-driving system respectively in order to project an image of a pattern formed on the first mask and an image of a pattern formed on the second mask onto the photosensitive substrate.